What you need to know about flow batteries

Flow batteries — big tanks of liquids that can store energy — were invented in the 19th century and investigated closely by NASA in the 1970s. Now, flow batteries are jockeying for a place in a growing energy storage market that will enable utilities to better manage the electric grid, provide back-up power and potentially work with the electric rates system.

The Electricity Energy Storage Association highlighted the development of flow batteries at its annual meeting in San Jose, Calif., on Tuesday, where a panel of public and private technology developers outlined their efforts to convince customers that flow batteries are cheaper and more reliable than a slew of other types of batteries and energy storage technologies that have been flooding the market as of late.

How they work

In contrast to a lithium-ion battery, where the energy-storing materials and electrolyte are enclosed in a cell, a flow battery’s electrolyte is stored in two tanks that are separate from the cell itself. The flow battery generates electricity when the liquid electrolytes, which are mixed with energy-storing materials, flow through the two-half cells and react with the electrodes in each side of the cell.

Flow battery makers like to point out that the use of external storage tanks means a flow battery system can be scaled up and down easily. Flow batteries are also rechargeable; the electrolytes can last a really long time, and typically use abundant materials — therefore can be a more affordable option. At the same time, flow batteries tend to be bulky, are not easily moved and the components have to be assembled on site.

Who buys them?

Flow battery developers are targeting some of the same customers that competing battery technology developers — from lithium-ion to lead acid makers — are aiming for. Utilities are sought-after customers, because they are facing a growing need to manage the variable supply of wind and solar electricity, and they can use batteries to store solar and wind energy and release it when demand peaks. But utilities aren’t easy customers to score, because they are known for their conservative (i.e. slow) approach to adopting anything new, especially when new technologies are pricey.

Businesses and consumers also are potential customers for battery companies. Batteries can store electricity from, say, a rooftop solar system, and then draw energy from the batteries after the sun goes down. Battery owners can even sell the stored energy to their utilities when electricity pricing is high if they are located in states that allow such a practice.

Funding sources

The federal government has doled out billions of dollars for all sorts of energy storage technologies for powering cars and building a smarter electric grid. Utilities have benefited from this largess by applying and receiving funds to carry out energy storage demonstration projects. Flow battery developers such as Primus Power and EnerVault are working on projects funded by the U.S. Department of Energy.

Venture capitalists are also occasionally funding flow battery companies. Last week, Primus Power announced it had raised $11 million in venture capital to help build a demonstration project in California and commercialize its technology. While the $11 million round shows there is private investor interest in energy storage, venture capitalists overall have been rather cautious about betting on energy storage, said Craig Horne, CEO and co-founder of EnerVault, which is developing iron-chromium flow battery, during the panel discussion on Tuesday.

“A lack of market pull is causing the investor community to approach this space with caution,” Horne said. “That’s true for stationary energy storage in general. There hasn’t been a big success in (flow) battery space, so it’s even more difficult to get the capital you need.”

The innovation

Flow battery developers are innovating around using different types of energy storing materials, such as iron, vanadium, zinc and bromine. Commonly the two electrolyte tanks hold different energy storing materials. But for vanadium flow batteries (an increasingly important and advanced type of flow battery) vanadium is used in both electrolyte solutions. The Electric Power Research Institute says vanadium flow batteries are a more mature technology than zinc- and iron-based flow batteries, which are mostly in R&D or field demonstration stages.

Prudent Energy, a vanadium flow battery maker, says one reason vanadium is a good solution is that using the same materials in both tanks means if the two solutions accidentally get mixed up, the battery won’t be permanently damaged. “Any mixing of the electrolytes ends up with something you have to clean up,” said Tim Hennessy, president of Prudent Energy, during the panel, but with vanadium flow batteries “We don’t have this concern. We lose some efficiencies, but there is no clean-up, so it can work continuously.”

Prudent builds its battery systems with 7 KW cells, and each system can be scaled up to 10 MW. The company has worked on projects in Japan and China, and it’s now participating in a project to demonstrate how its technology can work with solar systems from SunPower. Prudent plans to build a 100 KW system that can deliver 300 kilowatt-hours of energy, and also won a contract to install a 600 KW system at an onion processing plant in California.

Another company on the panel, RedFlow, offers a 5 KW (10 kilowatt-hours) zinc-bromine battery and is taking part in a government-funded smart grid trial in Australia, where the company will install 60, 5 KW systems, said Chris Winter, RedFlow’s chief technology officer and co-founder.